Modulating the magnetosphere of magnetars by internal magneto-elastic oscillations

Understand

We couple internal torsional, magneto-elastic oscillations of highly magnetized neutron stars (magnetars) to their magnetospheres.

• The corresponding axisymmetric perturbations of the external magnetic field configuration evolve as a sequence of linear, force-free equilibria that are completely determined by the background magnetic field configuration and by the perturbations of the magnetic field at the surface.
• The perturbations are obtained from simulations of magneto-elastic oscillations in the interior of the magnetar.
• While such oscillations can excite travelling Alfv\'en waves in the exterior of the star only in a very limited region close to the poles, they still modulate the near magnetosphere by inducing a time-dependent twist between the foot-points of closed magnetic field lines that exit the star at a polar angle $\gtrsim 0.19\,$rad.